(PRIOR ART STATEMENT)
As will be familiar to those having some knowledge of typewriters, it is possible with a typewriter to "print" a composite character made up of at least two other characters. This would normally be done by depressing one character key, backspacing and then depressing the second character key. Sometimes, with accented characters, the accent key is a so-called dead key so that to produce the composite accented character, it is only necessary to depress the accent key followed by the character key; but frequently it is necessary to backspace to produce an accented character. This production of composite characters is known as character overstrike.
There is a similar need to allow composite characters to be displayed on visual display units. Visual display units (VDUs) fall into two main types, one, such as that on the IBM 3270 PC/GX, in which characters are presented on a viewing screen by addressing an all points addressable refresh memory into which a bit pattern representing the character to be displayed is written, and a second type, such as that on the IBM 3277, 3278, 3279 and 8775, in which use is made of a coded character store containing coded representations of the various characters which can be displayed and a character generator which contains the actual bit pattern for those characters. The second type has the advantage that the bit patterns need be stored only once for each character code, no matter how many times that character needs to be displayed. Typically the character generator will consist of a read only memory, although a writable character generator store will allow different character sets to be loaded into the character generator. The display of the IBM 3270 PC/G has both a coded character refresh buffer and an all points addressable refresh buffer, and although the former is normally used for the display of alphanumeric characters, alphanumeric characters can also be displayed using the all points addressable refresh buffer in which case a composite character could also be displayed.
However it is not easy to display an overstruck composite character on a visual display unit of the type employing only a coded character refresh buffer in conjunction with a character generator.
U.S. Pat. No. 4,204,208 describes an arrangement in which two character generators are used with the ability to mix the bit pattern from one character generator with the bit pattern from the other character generator to produce a composite overlaid character. However such an arrangement is complex and does not readily lend itself to the situation in which the text is stored in the form "character 1", "back space", "character 2". As is explained in U.S. Pat. No. 4,464,730 text is conveniently stored in linear text stream form, with a formatter being utilized to read the sequentially stored text to load a cathode ray tube refresh buffer with appropriate character codes.